Multiplex transmission system



July 8, 1924. L. ESPENSCHIED MULTIPLEX TRANSMISSION SYSTEM Filed June 17, 1919 5 Sheets-Sheet l .i fm?" W FTM; i?? Z v @y 1.

INVENTOR.

' ATTORNEY L. ESPENSCHIED v MULTIPLEX TRANSMISSION SYSTEM I Filed June, 17 1919 5 Sheets-Sheet 2 INVENTOR.

L ATTORNEY.

July 8, w24. Y, '3,500,552

' L. ESPENSCHlED- MULTIPLEX TRANSMI S S ION SYSTEM Filed June 17 1919 5 Sheets-Sheet 3 v INI/ENTOR.

AHORA/Ey July 8 1924.

Lfz L.. ESPENSCHIED x MULTILEX TRANSMISSION SYSTEM Filed Jun@ 17, 1919 5 shegts-sheet 4 'A TTORNEY July s, 192.4. v1 1,500,552

@A ESPENSCHIED MULTIPLEX TRANSMISS ION SYSTEM FileqJune 1v. 1919 5 Smets-sheet 5 i INVENTR.

WQ ATTORNEY ltion,

Patented July 8, 1924.

Unirse STATES or HoLLraNEW iloxx,

A CORPORATION or ieaassz i-fArsurv ori-ica AMERICAN TELEPHONE NEW'v YORK vassiernoit 'ro MULTIPLEX franusms'sron SYSTEM.

Application flied June 17,

To all whom t Be' it known may concern.' that I, Lcorn EsrENscHrnD,

residing at Hollis, Long Island, in the county of Queens an State of New York,

have invented certain tiplex Transmission Systems, following is a specification.

This invention relates -to multiplex. transmission, and inor'eparticularly to multiplex systems in which carrier currents are use for the transmission of signals.

The invention comprises a system in which multiplex transmission is applied to a circuit of the four-wire type, that is, a`circuit in which one transmission line is use for transmission in one direction, and another .transmission line is used for transmission in the opposite direction. .It also comprises-a system in which multi lex'transinission is .applied to a transmission path which may e in part a twoway path and in part a four-wire one-way transmission path.

lt further comprises a transmission system in which signals from a plurality of signaling stations may be simultaneously transmitted in one direction to a distant station over a common signaling path, while signals from the distant stationy may e simultaneously transmitted over another si gnaling path in the opposite direction,..and inlividually transmitted to the corresponding signaling stations. 1

Another feature ot the invention consists in a transmission sy'tein in which a plurality of signals simultaneously' transmitted in one direction may Ibe separated at some desired point and se arately -transniit ted over one-Way lines to distant signaling stations, while signals frompsuch distant signaling stations may be'transmitted in the reverse direction over another set ot one-way lines, and then combined for simultaneous transmission over a common path.

The 'invention also. provides a transmission system in which 'a plurality of signals may bersimultaneously transmitted over a. common conductor to an intermediate staand there separated into individual channels so that? they may be transmitted. over individuallines to a distant point. The invention furthermore has reference to the. provision of a transmission circuit which is so arranged that frequencies lying within certain limits may be transmitted in of which the Improvements in Muli919. semi No. scarsi.

either direction over said circuit, while Jfrequencies lying Within other limits may be transmitted in one direction onl And the invention likewise concerns the provision of a transmission path having a two-Way repeater for the ampliication of transmission of a certain character, an a one-way repeater in parallel therewith for the amplification of transmission of a difierent character;

'In Order that the above results, as well as other aims ot the invention may be secured` it is proposed signaling lines, either or both of which may normally be used Jfor two-Way transmission, andl to superpose upon said lines a carrier system so organized that a plurality .ot carrier frequencies will be transmitted in one direction over, one line, and in the opposite direction over the other line. At the repeater points in these lines the usual two- Way repeaters, which are provided for the purpose of amplifying low frequency' signals. will not be interfered with, but oneway repeaters adapted for the amplification of the carrier frequencies will be'eoiinected in parallel with the two-Way repeaters, and will be arranged to repeat in the 'proper direction. Suitable means will also be provided to separate the low` frequency currents from the carrier currents, so that each type otutransmission is routed through the proper repeater. At terminal stations, the carrier frequencies may bese arated from the low frequencies, aiid\transmitted through suitable carrie-r apparatus which operates to separate the transmissions corresponding to the several channels and transmit them over separate receiving. and transmitting circuits to terminal lines-or stations. Where the terminal lines or stations are at points distant from the point at which the carrier apparatusis located, the separate transmitting audreeeiving lines may beI extended as one-way transmission lines so that each signaling station is provided with a tour-wire circuit. extending to the point at which the carrier apparatus islocated.

In certain instances it may be desirable to combine at some point the two sets o OppositelyY directed transmissions which are separately transmitted over the two sides ol' a 'tour-wire circuit and transmit them over a. two-way circuit to sonic distant point to utilize two low frequencyv Vapparatus through separate four-wire cirat which carrier apparatus is located. At this point the two sets of oppositely directed transmissions may be again separated, and the several transmissions of each set directed to proper transmitting `and receiving channels through which the carrier system is connected with individual terminal lines or stations. In this case also the" signaling stations may, if desired, be each associated with the carrier cuits.

The advantages attending the application of the four-wire principle to carrier transmission arise from the fact that theattenw ations of the high frequency carrier currents tainment of anything approaching an exact balance at the repeater points, thereby causing the repeaters to singif adjusted so as to give high ampliiications. By using four-wire circuits, the necessity for balancing the transmission lines at the repeater points is obviated, and the one-way repeaters may be adjusted to any amplitications desired, so long as the amplification around the four-wire path does not exceed the attenuation around said pathlby an amount sufficiently great to cause singing due to the unbalance existing at the terminal points of the Jfour-wire circuit.

The four-wire arrangement is similarly 'advantageous where it is desired to apply a carrier system to lines constructed entirely of cable. In this case the four-wire circuit permits the use of larger gains and the consequent.,greaterv separation of the repeater stations than would be possible with a twowire circuit. Furthermore it avoids the high degree of capacity balance required between twp-Wire cable circuits for high irequencies. This requirement may be obviated int e case of a four-wire circuit by grouping together respectively the east and west bound circuits, and electrically separating the groups by choosing non-adjacent. pairs of conductors in the cable, by shielding the two groups of conductors, or by employing separate cables 'for the two groups. These methods of separationorm no part of the present invention, however, and need not be described herein being'ully considered in the U. s. applicatifs of o. s. Blackwen, sa.

' the disadvantages incident to the use of our' wire circuits for ordinary low frequency transmission may be eliminated, namely: the utilization of two transmission paths for four-wire transmission as compared with the utilization of but one transmission pat-h for two-wire transmission. This requirement may not be a disadvantage in the oase of carrier transmission for the reason that, due to the severe cross talk conditions it is impossible to superpose carrier transmission channels of a given frequency upon more than one, orat most, a few low frequency lines of the same pole line, unless special transpositions or other changes are made to reduce cross talk. Since, for the reasons just given, all of the lines of a given pole line may not be available for two-wire' carrier transmission, there is no disadvantage'from the point voi' the invention, Figs. 4, 5 and 3 of which,

when placed side by side in the order named, constitute a circuit diagram of another embodiment of the invention, and

.Figs 6 and 7 of which illustrate forms of wave filters employed in connection with the invention.

Referring to Figs. fl, 2 and. 3 of the drawing, a four-wire carrier system is disclosed embodying special apparatus at four stations, A, B, C and D. Ordinary low frequency signaling lines. such as telephone lines ML, and ML2 are shown extending between stations A and B, while similar lines ML, and ML2' extend from stat-ions B to C, the two sets of lines being'interconnected at station B by means of repeater apparatus hereinafter described.

In order to provide for four-wire carrier transmission over 'the lines above referred, to, a common carrier ltransmit-ting circ-uit TL is branched from the line ML, at station A, through a filter HR. The filterV HF, is preferably an apparatus so designed as to modena freely allow the passa in the range employe in carrier transmission,A while substantially preventing the transmission of frequencies. lyingr outside or below saidrange, thereby insuring that the low frequency telephone currents which are ordinarily transmitted over the line will the not be divertedinto the carrier branch, but

will be transmitted to the terminal low frequency portion of the line LT1. Alon' frequency filter Ll"t is included in the low frcquency terminal portion of tlic line LT said filter being so arranged as to freely transmit the low frequency signaling 4current, such as telephone currents, while substantially prevent-ing the transmission of -hi her frequency carrier currents. These lters are of the general type disclosed in U. S. patents to George A. Campbell 1,227,113 and 1,227,114, dated May 22, 1917, and as set forth in said patents may be designed to transmit any desired range of frequencies. rThe high frequency filter HF1, may be of the type shown in detail in Fig. ,6 in which case it consists of a plurality of sections, each including series capacity and shunt inductance. The low frequency filter LF, may -be constituted structurally as indicated in Fig. 7, in which case it coinprises a'plurality of sections,eaeh having series inductance and shunt capacity.

In a similar manner a common carrier receiving circuit RL may be branched from the main Iline ML2 through a high frequency filter HF, in all respects similar to the filter HF While a low `frequency filter LF similar in character to the filter LF, may be inserted in the terminal portioirL'lQ'of the line ML2. The filter lll"4 prevents the transmission oflonv frequency current such as teleph ne currents to the carrier branch, While filtr LF4 prevents the transmission of carrier frequencies to the low frequency terminal a paratus.

A plura ity of low frequency signaling lines, as, for instance, telephone lines L L2, L and L are shown associated through transformer arrangements l1, l2, 13 and 14, with individual carrier transmitting channels TL)'TL TL., and TL4, as well as Withindividual carrier receiving channels RL HL2, RL3 and RL,. ln order that the individual transmitting and receiving channels may' be conjugate with respect to each otherl, that is, may be so arranged electrically tluat electroinotive forces arising in the one circuit will be substantially without effect in the other circuit, the transformers 11, 12A etc. are of the well known balanced type, and each of the linesl L1, L2 etc. are balanced by means of networks N N2, N,

and N,.

Each of the. individualtransmitting chan- -nels TB, TL2 etc. includes means -for trans lating low .frequency signaling currents re e of frequencies with- F or this purpose, modulating devices ceived from the corresponding low frequency line into liighfrequency oscillations for carrier transmission purposes, as Well as means for maintaining afrequency separation between the transmitting channels.

M2, M3, and M are inserted in the channels TLl, TL2 etc., these'modulating devices being of any type, 'well known in the art, but preferably vacuum tube modulators of the gener-.il character disclosed in the. S. application of B. lV.. Kendall, Serial No. 125,349,1iled .October 13, 1916, or in the United States application ofJolin R., Carson, Serial No. 157,413, filed March 26, 1917. Each modulator has impressed upon it, in addition to the modulating signal current, a carrier current having a frequency individual to each 'particular channel. These frequenciesmay be of any desired value, but should be so chosen as to maintain good frequency separation between the channels. For purposes of illustration, frequencies' of 10,000, 15,000, 20,000 and 25,000 are assigned to the channels 'l`L'TL' TL TL4 respectively.

filters, such as TF TF2, TF, and TF4 are -provided. These filters are also of the general type disclosed in the Campbell patents above referred to, and are so designed as to transmit a band of frequencies lying in the region of the carrier frequenc assigned to the corresponding channel, the band having a width sufficient to allow for the frequency variation due te the modulation of the carrier currents by the low frequency signal currents. .ln the case of telephone transmission, the width of'the band would besulistantially equal to the frequency range employed in ordinary telephone transmission.l ln order to amplify the modulating carrier frequencies, amplifying means TA TA?, TA, and TA, are prqs'ided in the .several channels. these amplifying means being of any well known typ'e, but prefer-' ably of the well known vacuum tube type. 1f desired. filters 'RFM T,F T,F, and '11154, may be included in the output'eircuits of the amplifiers for the purpose of further increasing the selectivity. These filters may be of the saine type as the filters 'l`l` TF,7 etc. A

(cining 'non' to the receiving portion of the 'carrier apparatus, it will be noted that the receiving channels RLQ, RL2 etc. contain .filters lili, R152,` RF, and RF4. These filters may be of thel suine type as the filters 'l`l"`.I T F, etc. and are so designed as to transinit the carrier frequencies ot' each channel as modulated by signalingcurrcnts at distiiiit stations. These dctcctois may be ol" any well lino-.vn type, but are preferably 0f the vacuum tube type illustrated and d escribed in thc application if .lohn R. Cal,

A any hig son, Serial No. 157,414, filed March 26,1917. The detecting or dcmodulating arrangement of the said application of lohn R. Carson, employs the so-called honiodyne method of detecting, thereby necessitating that'the carrier frequency characteristic of each particular channel be supplied to the dcmodulating arrangement of each channel. Accordingly, frequencies of 10,000, 15,000, 20,000 and 25,000 cycles are supplied to the demodulators D1, D2, D3 and D4. It will be understood, of course, that these frequencies are given merely for purposes of illustration, and that other frequencies may be assigned to the channels, the essential requirement being that for telephonie purposes the same frequency be supplied to the dcmodulator as that used at the originating station for transmission purposes.

In order to amplify' the low frequency waves which appear in the output c'ircuits of the modulating arrangements, amplifiers BA1, RAE, RA.d and RA4 are provided in the several receiving channels. These amplifiers may be of any Well known type, but are preferably vacuum tube amplifiers. In order to puri the low frequency currents from frequency components, low frequency filters such as R,F R,F2, R1F3, R1F4-arc provided in the output circuits of the amplifier. These filters may be of the same character as the low frequency filters LFI, and LF4 in the terminal lines LF, and LF2, and are of the general type disclosed in the patents to Campbell above referred to. The amplified low frequency signal waves from the several receiving channels., after being filtered,` are impressed through the transformers 11, 12, 13 and 14 upon the low frequency lines Ll, L2, L., and L4 respectively.

At the station B amplifying arrange.' ments are provided for the various currents transmitted over the main linee. Between the main line sections ML, and MLJ, the usual'two-way repeater empio ed for telephone transmission is provi ed, said repeater including an amplifying tube AF., for repeating in one direction and an ampliltying tube AW, for repeating in theopposite direction. By means of this repeater the low frequency signaling current such as telephone currents may be transmitted over the mainline sections MLl'and MLl in both directions.

Since carrier frequencies are transmitted over the main line sections ML,A and ML] in the direction from station A. to station C only, a one-Way repeating arrangement AlE is provided for amplifying the carrier oscillations. T his amplifier is connected in parallel with the two-way repeater just described through carrier circuits TJ. and T,1, branched from the main line sections ML, and M131 at the repeater point. In or- Lacasse der to prevent the transmission of carrier frequencies to the 4two-way repeater filters LF2 and LFE are provided, these filters being vof the same type as the filter LF1 already described. In order to prevent the transmission of low frequency signaling cirrents through the circuit of the one-way amplifier A1E, filters HF, and HF.,r are provided in lthe carrier circuits TIL and TlL. These. filters are of the saine type as the filters HF1 previously described, and. are so designed as to freely transmit carrier frequencies While suppressing other Afrequencies. In order to balancev the filters LF2 and LF. with regard to the two-Way repeater, devices LF3 and LFn are provided in the, balancing circuit of said. repeater. These devicesmay either he duplicate filters of the same type as the filters which they are intended to balance, or they may be networks so designed as to have electrical characteristics simulating those of the lters.

The one-way repeater AIE is a duplexl two-stage arrangement comprising amplifying tubes 21 and 22 connected in tandem with each other and in parallel with tandem tubes 23 and 24;. The amplifying tubes and the circuits therefor are so balanced that variations impressed upon the input circuits of the amplifying arrangement produce no change in the current flow in the common portions of the parallel circuits of the two sets of tubes, so that the resultant effect is that of current variations flowing in series over the portion of the amplifying circuit shown in heavy lines. The result of this arrangement is that minute variations impressed upon the amplifying arrangement through the transformer 25 appear upon the output side of the transformer 26 as amplified variations which are faithful copies of the original variations, since the balanced arrangement of the amplifier tends to minimize the distortion which might otherwise occur.

The repeating arrangements associated with the mainline sections ML2 and ML2 are in all respects similar to those already described, and comprise a two-wav repeater, including amplifiers All)2 and AW2 for amplifying, low frequency, signaling currents in either direction, and a one-way amplifier AIW for amplifying carrier frequencies transmitted from station C to station A. The one'way amplifier A,W is connected in parallel-with the two-way repeater through the carrier branches RIL and BIL'. In 0rder to maintain a soparation'between the carrier frequencies and the ordinary signalmg frequencies, filters l-l'F,K and HF( are included in the branches R,L and R1L' and filters LF5 and LFS are included in circuit with the two-way repeater. These filters are similar to the corresponding filters of the other repeating arrangement. ln orden to balance the filters LF, and LFS with'respect to the two-way repeater, balancing arrangements LF, and LFG are provided, While balancing networks MN2 and MNZ are included in the balancing circuits of the repeater for simulating-the mainline sections, and other apparatus external to the filters LF5 and LF. 7

The one-wa repeater A,W comprises tandem tubes2 and 28 connected in parallel with tandem tubes 29 and 30.- The repeater is associated with the carrier branches R,L and`R,L through transformers 31 and 32. This oneaway amplifying arrangement func tions ina manner similar to that ofl the amplifying arrangement A,E already described. y

' Coming now to the apparatus at station C, the arrangement of the carrier apparatus isin general the same as that alrea'dy de scribed with reference to station A, and simi lar pieces of apparatus are designated by the same reference characters as used in connection with the apparat-us at station A,.with the exception that 'in each. instance-the reference character is primed. The larrangement at station C differs from that at station A in that the common carrierreceiving branch RL is associated with the line section ML] and the common transmitting branch- TL is associated with the main line section ML2', this arrangement being just` the reverse of that at station A. The ap, paratus in the several carrier transmitting channels, such as TL,, TL2 etc., ,and in the several carrier receiving channels RL, and RL, etc. is similar to that at station A, and-need not be further described?,

As illustrated in Fig. 3, the'terminating transmission lines L,, L2', La and L4 do not terminate at the station C, but terminate at a distant station D, at which point they are associated by means of transformers 1l', l2', 13 and 14 ywith the carrier ref eciving and transmitting channels, which,

instead of being local circuits at the station"y C are extended to the distant station-D. The terminal transmission lines are also balanced at station D by means of artificial lines or networks N,, N2', NQ/ and Nj. 'It will be seen, therefore, that each low frequency transmission line, such as L,, L2 etc. is connected with station C by means of 'a four-wire circuit so that transmission takes place bein-ecu the station C and a given terminal line at station B in one direction over a transmitting channel, and in the other direction over a receiving channel. If, however, the transformers und artificial lines be brought up'io the saine pointl as the carrier apparatus at station l, and the conductors .of the several transmitting and receiving channels be correspondingly shortened, the

arrangen'ieut is identical with that shown at l station A in' Figure 1. It will be apparent that these tWo arrangements of the terminating carrier apparatus are interchangeable, and either the four-wire arrangement or the arrangement shown at station A may be' used at either station, the two forms of apparatus being shown differently at the two stations forthe purpose of illustrating two possible modifications, WithoutV unnecessary duplication of the drawing.

F urtherdetails of the invention will now be clear from a description of the operation, 'which is asv follows: Low frequency signaling currents, such as telephone currents, coming'in over the terminating line LT, are passed through the filter LF over the line section ML, and through the filter LF2 to the two-way repeating apparatus. These currents upon being amplified by the amplifier AE, of the repeating apparatus are transmitted through the filter LFgl' over the line section hILfand throu h the filter l LF, to the terminating line L Transmission in the opposite direction takes place is used to amplify the si als. It will be seen, therefore, that the ine sections ML,

and ML, operate as two-way transmission lines for ordinary signaling currents.

Low frequency signaling currents coming in over the line L, at station A are-trans mitted through the transformer 11 and over the carrier transmitting channel TL, tothe modulator M, where they are combined with currents of the carrier frequency assigned to this channel, forl instance 10,000 cycles, and the carrier frequenc currents are modulated inaccordance wit the signaling cui rents. The modulated carrier currentsare then transmitted throu h the filter TF are amplified by the ampli er TA, and are then transmitted through the filter T, F, to the common carrier branch TL. In a similar manner, currents from the lines L,',L`L are transmitted through the corresponding carrier channels where they modulate the corresponding carrier frequencies, and the modulated' frequencies arev impressedupon the common carrier branch TL. The several modulated carrier ,frequencies are now transmitted through the high frequency fil Athe carrier frequencies passing through the latter filter to the one-way amplifier A,2. The several modulated carrier frequencies are amplified by the one-way amplifier, and are transmitted through the transformer 26 and the high frequency' filter HF, to the main line section M11. After being transmitted over said main line section to station C, the carrier frequencies are again separated from the low frequency-si alin currents by means of the, lters L 1 an The several carrier frequencies are trans mitted through the filter mon carrier circuit RL', and the modulated carrier currents corresponding to the sev-t eral channels are selected by means of thev filters RF1', RF1 etc. and'im ressed upon the detectors -ormodulators D1', D2 etc. At the same time, unmodulated oscillations of the carrier frequency assigned to each channelare impressed upon the several modulators, and in a manner well known in the art the low frequency signalin currents, by means of which the carrier requencies at station A were originally modulated appear in the output `circuits of thedemodulators. These lowI frequency currents, after being amplified the amplifiers RA1, RAZ etc. are transmitted through the low frequency filters 11,1%- ,iRlF 2 etc., and over the onevWay lines RLl, HL2 etc. to station D. At

this station thelow frequency currents are transmitted through the transformer and artifsial line larrangements to the terminating ow frequency lines L1', Lzl, La and Low frequency si aling currents coming in over these four ow frequency lines are transmitted through the 4transformers 11', 12 etc. to the-carrier channels TD1, TL," etc., and over these carrier channels to station D. By means of the carrier apparatus in the channels, these low frequency. sivnaling currents modulate the corresponding carrier frequencies, are amplified and filtered 'and then simiiltaneously impressed upon the com mon carrier transmitting circuit TL. From this point they are transmitted through the high frequency filter HF, and over the main line section ML2 to station B.

At station B the modulated carrier frequencies are separated from the low frequency signaling currents transmitted between line sections LT2 and LT; by means of the filters HF5 and LF. The low frequency currents are amplified by means of lthe two-Way repeater comprising the amplifiers AE, and AWT The carrier fre uencics, however, are transmitted through t e filter HF; to the amplifying arrangement A,W. After being amplified by means of this amplifying arrangement, they are transmitted through the transformer 31 and the high frequency filter HF, to the line section liiiz.

The several modulated frequencies after being transmitted over said main line li 2 are separated from the low fre uencyJsignaling currents by means of the fi ters F4 and HR. the carrier frequencies being transmitted through the latter filter to a common receiving circuit RL at station'A. The several modulated frl uencies are .now separated by means of the l being detected the resultant signaling currents are amplified, filtered and transmitted through the carrier receiving channels R111, RL, etc. to the terminating lines L1, L2, Ls

It will thus 'be' seen that the carrier'frefi'iuencies. 'are transmitted simultaneously Bver twonain transmission. lines comprising a four-wire circuit, said transmission lines operating as one-way transmission circuits. At the same time the two transmission lines comprising the four-wire circuit for carrier purposes are used as two-wire lines fortwo-wa low frequency transmission. The severa low frequencies may also, if desired, be transmitted over four-Wire circuits to the carrier apparatus as shown in ters RF, and RF and after' Fig. 3. It will also be noted that the carrier apparatus at both the terminal stations4 and the repeater points is separate and distinct from the low frequency transmission apparatus, and may be su erposed upon low frequency transmission ines -without an material change in the latter, and without a fecting the operation of the latter in any essential detail.

A mpdilied arrangement in which a fourwire transmission circuit is combined with' a two-)Wire transmission circuit is illustrated in F i standing the diagram, the three sheets in'- cluding these' ngures 4should be laced side by side in the order named. In t is modification carrier transmissiony between stations B and C takes place over a four-Wire cir-- cuit, while carrier transmission between stations A and B takes place, over a two-wire circuit. The apparatus at stations C and -D is, of course, identical with that described in connection with the previous modification and, as stated in connection with said modication, the terminal low frequency lines,

4, 5 and 3. For more readily underline section M11 and ML2 in the same' trated as the main line section MLI, the association of the opposite ends of the repeaters with the transmission circuit between stations A and B is somewhat different from that previously described. In order carrie?` Leonesa to se arate the earrier frequencies from the low requency sivnaling currents, a common.

1B 1s branched from thel quency filter LF2, similar to the filter LFZ,

is inserted between the main line section ML1 and the two-Way repeater comprising amplifiers AE1 and A'Wl.

The common carrier circuit C1B is asso- I' ciated vvith common 'carrier transmitting 1 and receiving circuits TIL and RlL through arrangement I-IIF1 which may a balanced transformer arrangement 33. In order that the circuits RIL and T1L maybe made conjugate with respect to each other, lthe filter HF is balanced by means of a balancing arrangement HlFz which may be,`

either a duplicate filter or alnetwork whose electricalv characteristics simulate those of the' filter. The mainline ML, and other apparatus external to the filter'I-IF,1 may be balanced by meansof a network MlNl. The circuits T1L and RIL are associated with the one-way amplifiers, A,E and A1W through transformers 25-and 31 respectively.

At station A, the carrier frequencies are separated from the low fre uency signalingI currents transmitted over t e line ML,l by means of lter LF1 and HF, which are sinular. inail respects to the corresponding lters at sta-tionA ofFig.` l. The carrier frequencies upon being separated by means of this filtering arrangement' are impressed upon the common carrier branch CB which is associated by means of a balanced transformer arrangement 10 with common carrier transmitting and receiving circuits TL and Ii-L. In order to render these twocircuits conju ate Withvrespect'to each other, 'the filter Fl is ,balanced-by abalancing be either a duplicate .filter or a network simulating the electrical characteristics of the filter. A balancing network MN is also provided to balance'the main line MLl and the other apparatus external to the filter HF-l.

Carrier transmitting .channels TLl, TL2, TL3 and TI.-4 are associated with the common carrier transmitting circuit TL and include modulating, amplifying and filtering apparatus similar to that described in connection with Fig. 1. In a similar manner, carrier receiving channelsI RL1, BLZ, RL3 and RL.l are associated'with the common carrier receiving circuitI fRL, and :contain high frequency filters, demodulating 'apparatus, amplifying' apparatus and low frequency filters similar to the corresponding apparatus described in connection with the station A of Fig. 41. The several transmitting und receiving channels may be connected to the terminating low frequency lines L1, L2, L3 and Lc1 either at the station A directly or at a distant station to which channels TL1 and TL2 etc., and b of the carrier apparatus alread fied by sai they are extended as four-Wire circuits in a manner similar to that shown at station D. In either case the means for associating these circuits comprises transformers 11, 12, 13 and 14 and thc balancing artificial' lines -or networks N1, N2, N3 and N4.

The operation is as lfollows: Low frequency signaling currents may be transmit- .ted between the terminal lines LT1 and LT1 in either direction over the transmission line sections ML, and ML1through the low frequency filters and through the two-way repeater comprising ampli ers AE, and AW'I. Low frequency signaling currents incoming from the terminal lines L1, L2, L, and L are transmitted through the transformen l1, 12, 13 and 14 to the carrier transmitting means escribed these low frequency'v currents mo ulate oscillations of the carrier frequenc assigned` to each channel, such modulated" oscillations being am lified, filtered and simultaneously impresse upon the common transmitting circuit TL. The several modulated frequencies 'are then Atransmitted through the transformer 10 over the carrier branch OB and through the 'high 'frequency filter. HF1

, to themain line section MLI.A

After being transmitted over the main line section M331,4 the several carrier frequencies are separated :it-station B by means of filters LF2 and HFQ, and 'are impressed upon the common carrier branch C1B.

i From lthis circuitthe several frequencies are transmitted throughlthe transformer 33 to the common transmitting circuit TlL and thence through the`transformer 25 to the one-Wayrepeater AlE. After being amplirepeater they are' transmitted through the transformer 26, through the high frequency filter HF2 and over 'the main line section ML to station C. At station yC the carrier frequencies are separated from the low frequency signaling cur-v rents and are then selected into the channels, detected, modified, filtere 'and transmitted to the terminal lines L1, L2 etc. in a manner already described. v A

Low frequency signaling currents incoming from the terminal lines L1', L2 etc. are transmitted to the carrier transmitter channels TLI, TLZ etc., and after modulation, amplification and transmission throu h the filters in a manner already describe they are `impressed upon the/common transmitting circuit TL. From this circuit lthe 'modulated carrier frequencies are transmitted through the filter HF( and over the main line section ML2 to station B.

At station B the high frequency cari-ier currents are separated from the low frequeneusignfaling currents-by lters LFS and I-'IF5 andfare'then impressed through the transformer 32 upon the one-way amplifier JV. After being amplified the modulated carrier frequencies are transmitted through the transformer 31 and over thev common receiving branch PHL to the common carrier branch C1B. l

Afterpassing through the filter 'HP2 and overthe main ine section ML-the'carrier frequencies are separated from the low fre-V qi'ency currents at station A by means of the' filters LF, and HF the carrier frequencies.

thermore, while the invention has been gen erally described as applied to telephonie transmission, the same principles are v'equally applicable to telegi'apliic or other forms 'of signal transmission. Thus, the low frequency terminal lines LT1, L L2, Lsand L4 may be either telegraph or telephone lines,

or combinations of both.

In general, while the invention has been illustrated as embodied in certain specilic forms, it will be obvious that the eneral principles herein disclosed may be em odied in many other organizations widely ldifferent from those illustrated, witlout rting from the spirit of the invention as e 'ned in the following claims.

What is claimed is:

1. In a transmission stem, a. pair of transmission paths exten ing between vtwo geographically separated oiices, one path be-,

ing adapted for transmission in one direction and the other path being adapted for transmission in the opposite direction, a plurality ot' signaling stations, signaling chan' nels from each station superposed on one of said paths for transmission in one direction,` and separate signaling channels from each' station imposed on the other path for transmission iii the opposite direction, the attenuation ot said paths being sutliciently great to require amplification of energy transmitted thereover.

2. T n a transmission s stein, a pair of transmission paths exten ing between two geographically separated offices. one path being adapted for transmission in one direction' and the other path heilig adapted for transmission in the opposite direction, a plurality of signaling stations, signaling channels from each station superposed on one of said paths for transmission in one direction, and

separate signaling channels from each station imposed on the other path for transmission in the opposite direction and oneway repeaters in each path for amplifying 'the transmitted currents corresponding to each channel.

3. In a'transmission system, a pair of transmission lines interconnecting two geographically separated oiiices, the attenuation of said lines being suliiciently great to requireampliication of energy transmitted 'thereover, a plurality `of terminating lines,

and means for simultaneously interconnecting corresponding ends of both of said transmission lines with each of said terminating lines so that transmission may take place from all of the terminating lines over each transmission line of said pair.

4. `In a transmission system, a pair of transmission lines interconnecting two points, a plurality of terminating lines, means for 'connectin each of said terminating lines with one o said transmission lines so that transmission from said terminating lines will-take place over said transmission line in one direction only, means for connecting said terminating lines to the other transmission line so. that transmission over said other line to said terminating lines will take place in the opposite direction only.

'5. In a transmission stem a. pair of transmission lines extending between two geographically separated oiiices, a plurality of signaling stations, means for associatin each of said signaling stations with one o said lines so that transmission between said stations. and said line will take place over said line in one direction only, and means for connecting the other line with each of said stations so that transmission between said line and said stations will take place over said lin'ein the o posite direction onlV, the attenuation of said) ines being suiiiciently great to reqluire amplification of energy transmitted t iereover.

6. In a transmission system, a plurality of signaling stations at one point, a correspending plurality of signaling stations at a distant point, a common transmission line extending from said first mentioned point `to an intermediate point, a pair ofvtransmission lines for opposite transmission between said intermediate point and said distant point, said pair of transmission lines, each having attenuation sufficiently great to i'equireampliiication of energy transmitted thercover, means whereby transmission in both directions may simultaneously take place between a plurality of signaling stations at said first mentioned point and said intermediate point over said common transmission line, means for separating opposite transmissions at said intermediate point and directing simultaneous transmissions in one direction over one line extending to said po'site directions between said intermediate' otiice and a distant otlice, said pair of transmission lines` each having attenuation suficiently great, to' require amplification of energy transmitted thereover, means for transmitting a plurality of oppositely'directed transmissions over said two Way line between said first oiiice and said intermediate oflce, means for separating oppositely directed transmissions at, said intermediate otiice andtransmitting -simultaneous transmissions having the same direction over-one of said pair ofiransmission .lines to said distant oiioe, while transmitting simultaneons transmissions opposite in direction from said' distant otiice to saidiintermediate otiice over the other of said pair of transmission lines. i

"8. In 'a transmission system for transmis sion between -two points, a two Way transmission-'.line'extending from one point to an intermediate point, av pair of one-way transmission lines extending .from said intermediate point to-a distantpoint, means for interconnecting said one -way lines at said intermediate point with said two-way line, means for associating a plurality of two-way channels with said two-way line at said first point and means for associating a plurality of one-way. channels with each ot' said one-Way lines at said distant point. A A

9. In a transmission system for transmis` sion between two points, a two .way transmission line extending from one point t0 an intermediate point, a pair of one-way transmission lines extending from said intermediate point to a distant point, means for interconnecting sai: one-way lines at said intermediate point with said two-way line, means for associating a plurality of two-way channels with' said two-way line at said rst point, means for associating a plurality of one-way channels with each o said one-way lines at said distant point, and means for associating pairs of one-way channels associated with said oneway lines with terminal transmission lines.

10. In a transmission system, a plurality of terminating transmission lines, a pair of one-way lines adapted for multiplex transmission extending between two geographically separated oices and associated'with said plurality of terminating lines, said onewaylines having attenuation sufficiently great to require amplification of energy transmitted thereover, means whereby outgoing transmission from each of a plurality of said terminating lines may be simultaneously transmitted over one of said one-way lines, n'd means whereby 'simultaneous incoming` 'transmissions for said terminating lines may be transmitted over the other one- Way line and separately directed to the proper terminating lines.

11. In a transmission system, a plurality of terminating transmission lines, a pair of one-way lines adapted for multiplex transmission extending between two eographically separated ofiices and assoclated with said plurality of terminating lines, said oneway lines having attenuation sutliciently great to require amplification of energy transmitted thereover, means for translating low frequency signaling currents from said terminating lines into high frequency carrier oscillations having fr uencies characteristic of individual terminating lilies, means .for simultaneously transmittmg the translated frequencies over one of said oneway lines in one direction, means whereby a plurality of 'high frequencies may betransmitted over the other one-way line inthe 'of signaling stations at a distant point, terminat'ing one-way lines extending from each of -saldl stations .to said intermediate point, means at the intermediate point for associating said'terminating lines with one of said' multiplex lines whereby transmissions from said stations may be transmitted in one direction overl said terminal lines and then simultaneously transmitted in one direction over said multiplex line, other terminatin one-way lines extending from the other mu tiplex line to said signaling stations for transmission in the opposite direction, and means whereby a plurality of transmissions over said other multiplex line in such opposite direction may be separated at the intermediate point and separately transmitted over said other terminating lines to said signaling stations.

13. In a transmission system, a plurality of signaling stations. transmission lines for transmission in one direction extending one from each station to a distant point, the currents transmitted over sai d lines to said point being sutiiciently attenuated to require mission in amplification, transmission lines for transthe opposite direction extending from said distant point to each of said sta- Ao isigualing stations, transmission 'nes for transmission in one direction extending one from each station to a distant point, the currents transmitted over said lines to said point suiiiciently attenuated to require amplitication, transmission lines for transmis- 'sion in the opposite direction extending Afrornsaid distant point to each of said stations, said means being so arran A 'that' n transmissions to and from said stations will be se arated so as to take place over separate es. v'

15. In a transmission system, a lurality of signaling stations, transmission es for transmission in one direction extending one from each station to a distant point, transmission lines for transmision 1n the opposite direction extending from said distant point to each of' said stations, a pair of multiplex lines having suilicient attenuation to require amplification of currents transmitted thereover means whereby the individual lines associated with said stationsfor transmission in 'one direction may be connected with one end of one of said multi lex lines, and means whereby the individu lines associat- -edwitli said stations for transmission in the interconnected that op op 'te direction may -be connected with one end of the other multiplex line.

16.- In a transmission system, a pllllllrality of signaling stations, transmission es for transmission in one direction extending one.

romeach station to a distant oint, transmission linea for transmission 1n the opposite direction extending from said distant int to each of saidstations, a pair of multiplex lines havi suicient attenuation to re uire ampliticatxon of vcurrents trans. mit therecvcr, means whereby the individual lines associated with mid stations for transmission in one direction may be con nested with one end of one of said multi lex lines, and means whereby the individual mes associated with said stations for transmission in the opposite direction may be connected with one end of the other multiplex line, said multiplex lines having their opposite ends so posits transmissions thereover will be maintainedseparate.

17. In a transmission system, a multiplex transmission line over which a plurality of transmissions may take place simultaneous'- ly, said transmission line extendi between two ints, a plurality of. termina lines extending from one of said points to distant points, currents transmitted over said ter-- minal lines to said distant point being sufticiently attenuated to require amplification, means for associating said terminal lines neccesswith said multi lex line, and means to separate into indivi ual terminal lines for transmission to said distant points without change in frequency the simultaneous transiliililssions takingl place over saidmultiplex 18. In a transmission system, a transmission line,:means' associated with said line permitting the transmission oflow frequencies in both directions over said line, and

means associated with said line whereby fre-- que'neies above the voice ran e may be transmitted over said line in one irection only.

19. In al transmission system, a transmission line, means associated with said line whereby frequencies within a certain range may be transmitted over said line in both directions, and means associated with said line whereby frequencies above the voice range and within a range different from said first mentioned range may be transmitted over said line in one direction only.

20. In a transmission system, a transmission line, a two-way repeater'in said line, and a sin le' one-wa repeater associated with said line in para lel with said two-way repeater, lmeans whereby only frequencies within the voice range willbe transmitted through said two way repeater, and means whereby only frequencies above the voice range will be transmitted through said oneway re atei'.

21. n a transmission system, a pair of independent circuits, one circuit belng adapt'- ed for transmitting signals to a distant point and the other circuit being adapted to receive si als from said distant point, said circuits aving attenuation suiciently great to reuire amplification of currents transmitte thereover, a plurality of signaling stations, signaling channels from each sta-2 d upon one of said circuits tion supe for transmission to a distant point, and sepcrate si sling channels to each station supe upon the other of said circuits for transmission from said distant point.

22. In a transmission system, a transmission ath over which carrier frequencies may transmitted'to a station, av second lll station, a lurality of lines, one correspon i to escll carrier frequency and extending be Ween said stations for transmission in one direction, currents transmitted over' said lines to said second station sucientiy .attenuated to require amplification, and an equal number o lines extending between said stations for transmission in the opposite direction, all' o said lines bein interconnectcd at said first mentioned st tion with said transmission ath.A

In testimony w ereof, I have s ed my name to this apecication this 16t day of June, 1919.

LLOYD ESPENSCHIED. 

